The present invention relates to an electrode for an electric cell and in particular for a fuel cell using a liquid electrolyte and designed to consume a first oxidizing agent gas such as oxygen or atmospheric air, for example, and a second reducing agent gas such as hydrogen. It applies in particular to a filter-press type cell such as described in U.S. Pat. No. 4,002,493, for "A fuel cell structure and system, more particularly for a carbon fuel and atmosphere air".
In such a cell, current is transferred between neighbouring electrodes belonging to two adjacent cells by means of electronically conductive impermeable corrugated current collectors which are in contact with the electrodes. Contact may be maintained by simple pressure or by bonding at points of contact or along lines at a spacing of about one millimeter.
Further, the impermeable structure of such bipolar collectors cooperates with each electrode to define passages for the corresponding reactive gas, with the passages as a whole constituting the gas compartment of said electrode.
Each fuel cell electrode must include at least one porous layer (here referred to as a catalytic or active layer) having one free surface in contact with the electrolyte and fed with a reagent gas via its outer surface. More precisely, the catalytic layer must be able to promote the electrochemical process and for this purpose it must be sufficiently porous to form a liquid-gas interface between the electrolyte which enters via the wetted surface and the reagent gas which arrives via the other surface. Reaction takes place at the line of contact (referred to as the "triple line") between the gas-electrolyte interface and the surface of the porous layer. The electrode must also include--at least in the vicinity of the triple line--an electrocatalyst capable of catalysing the oxidation or the reduction reaction, as the case may be, of the reagent gas. The electrocatalyst is in contact with an electron conductor which is itself in continuous electrical connection with the current collector means which serve to transport the electrons which are produced or consumed in the reaction.
A catalytic layer therefore generally includes: a catalyst, an electron conductor (in the majority of cases, a metal or carbon) and, when neither the catalyst nor the electron conductor forms a continuous solid, a binder which, in most cases, is hydrophobic.
In prior art electrodes, to maximize the extent of the triple line, it was sought to make the liquid electrolyte enter the catalytic or active layer. To do this, the materials used for said layer were compounds which made it, in the aggregate, absorbent or, more precisely, wettable; when the binder was hydrophobic, this was achieved by using a sufficiently small quantity of binder relative to the quantities of other, absorbent, materials used in the layer. Generally, less than 50% of the layer as a whole constituted binder, and in most cases less than 40% and even less than 30%. This is the case, for example, of the catalytic layer described in U.S. Pat. No. 3,553,022 issued on 5th January, 1971, in which the catalytic layer may thus include 30% by weight of polytetrafluoroethylene which acts as a binder and 70% of platinum which acts as a catalyst.
However, with a catalytic or active layer which, in the aggregate, is wettable, the electrolyte finally enters completely and soaks the layer throughout, thereby preventing the reagent gas from entering, and consequently causes the triple line to disappear completely. To remedy this situation in prior art electrodes, a second porous layer is associated with the catalytic layer on the gas side, said second porous layer generally being referred to as a barrier layer and having the property of preventing electrolyte from entering its pores because the barrier layer is hydrophobic. Hence, the liquid-gas interface is stabilized at the interface between the two layers.
The barrier layer must be porous enough to allow reagent gases such as hydrogen or air to reach the catalytic layer by diffusion, while nevertheless preventing the electrolyte from flowing in the opposite direction towards the gas compartment of the electrode; however, it must also allow electron transfer between the catalytic layer and the collector.
To do this, such a macroscopically homogeneous layer must therefore include a material which is hydrophobic and therefore electronically insulative together with an electronically conductive material. The two essential functions of such a barrier layer therefore appear to be in contradiction with each other: if the hydrophobic material content is high, the electron conductivity is low and therefore so is performance; if the conductive material content is high, the material is insufficiently hydrophobic and this causes relatively rapid penetration of electrolyte and therefore a short service life.
In known electrodes, it is impossible to solve such a contradiction. Therefore, to overcome such disadvantages, electrodes have been made which include, in addition to the catalytic layer, a barrier layer with a high content of hydrophobic binder or a barrier layer which contains only hydrophobic binder, said electrodes also including an extra component of low binder content or constituted even by electronically conductive material only so that the electrode has sufficient electron conductivity to compensate for the poor conductivity of the hydrophobic layer. These electrodes therefore include, for example, a mesh or an expanded metal part, a graphite or carbon fabric, etc., imbricated in the barrier layer and/or in the catalytic layer, for example.
However, such known electrodes have a number of drawbacks.
Firstly, their complexity makes them thick, which results in bulky electric cells. Further, these electrodes are rather rigid because of their third conductive material content. This makes them mechanically fragile especially in filter-press type structures, such as in the above-described U.S. Pat. No. 4,002,493, and gives rise to interconnection difficulties in said structures.
Even in the case where these electrodes have only two layers, the fact that the catalytic layer, which is hydrophilic and therefore has a relatively low binder content, makes it particularly fragile and likely to fracture when the electrode is being handled and when it is subjected to mechanical or thermal strains and stresses, etc., during assembly of the cell and during operation, thus increasing the cost of the cell and shortening its service life.
In the case where a third component is provided e.g. a carbon fabric within which the catalytic layer is contained, said catalytic layer causes the electrode to be heterogeneous, which can give rise to a lack of cohesion in the electrode during operation, in particular because of the different coefficients of expansion of the binder and the material which constitutes the third component. Further, the hydrophilic nature of the carbon fabric, which is also in contact with the barrier layer, gives rise to conditions of particularly rapid inhibition in the active and barrier layers in the neighbourhood of their interface. This considerably shortens the service life of the electrode. It must also be observed that since the active layer takes up only the empty spaces between the meshes of the third component, the effective current density which the active layer transports is clearly higher than the apparent average current density. This increases polarization and reduces the service life of the electrode.
Further, the appreciable thickness of known electrodes requires them to have a highly porous structure which, in general, must be produced by pore-forming products during electrode manufacture. Subsequent removal of said products may be incomplete and the products may give rise to a heterogeneous structure. There is also a danger of thereby forming macroporosities which are liable to alter the operation of the electrode e.g. by promoting electrolyte weeping.
Lastly, such electrodes turn out to be relatively difficult to produce, especially on an industrial scale, and therefore their cost price is high.
Preferred embodiments of the present invention mitigate the above-mentioned drawbacks, particularly by providing electrodes whose service life is appreciably increased relative to that of prior art electrodes.